Slot wedges are strips of electrically insulating material, used to retain conductors in the coil slots of stators of dynamoelectric machines such as generators and motors. Prior art slot wedge structures have included phenolic resin impregnated, flat, Kraft paper sheet laminates. However, when subjected to temperatures on the order of 100.degree. C., after several years use, in large generators and motors, some shrinkage of the Kraft paper laminates was encountered. In addition, the Kraft paper-phenolic wedges had poor interlaminar shear length, and were abrasive to the inner surface edges of the iron stator teeth during the wedge driving operation. Asbestos-phenolic slot wedges have found wide acceptance, having good stability and lubricity characteristics, but the use of asbestos is now considered to be a potential health hazard.
White, in U.S. Pat. No. 3,437,858, tried to remedy shrinkage and shear strength problems, by providing a polyester resin impregnated, parallel glass fiber, extruded slot wedge, having a core of low shear strength. This structure included at each end, a metal or glass fiber tube, rod, tape or cord, having a very high shear strength. Thus, the highest shear strength was at the portion of the wedge that contacted the inner surface of the stator teeth. This wedge was faced with a 5 to 30 mil thick tape of wrapped woven glass, which provided a high transverse bonding strength, and allowed increased driving pressure during wedge insertion. The tape covering also added to the shear strength of the wedge, since 1/2 of the glass fibers were transverse to the slot wedge core fibers. Such wedges would, however, still be abrasive to the inner surface edges of the iron stator teeth, during the wedge driving operation.
Balke, in U.S. Pat. Nos. 3,735,169, provided plural players of Kapton polyimide film, or Nomex (poly 1,3-phenyleneisophthalamide) polyamide, high density, fibrous sheet, laminated together with adhesive, to form flat composites. These sheets, with applied adhesive, were placed in a clamping fixture, and then laminated, to cure the adhesive. They formed rigid plastic wedges, with high temperature dimensional stability, having the desired channel shaped slot wedge configuration, without using a supporting core. Such a construction, however, relies upon the thin adhesive layer for rigidity, and would provide wedges which could still allow substantial conductor displacement and vibration. This type of wedge would be practical for small appliances, where coil forces are about 1 lb./inch length of slot wedge, but not for large dynamoelectric machines, with coil forces of about 100 lb./inch length of slot wedge.
What is needed, is a strong wedge, able to prevent conductor displacement and vibration, and resist shear stresses, shrinkage, and bowing caused by the pressure of the wedged conductors and heat. The wedge should, very importantly, also provide a compressible iron engaging surface of considerable resiliency and lubricity, which would not abrade the inner surface edges of the laminated stator teeth during the wedge driving operation.